1. Field of the Invention
The present invention relates to current sources and, in particular, to a high-speed, fully-isolated, high-power current source/sink that uses a current mode op amp to drive a power MOSFET as the current regulating device.
2. Discussion of the Prior Art
For ideal operation, certain electrical systems, e.g. integrated circuit tester environments, require a current source with highly stable signal characteristics. However, as described in greater detail below, conventional current sources typically experience stability problems due to loss of phase margin. In order to guarantee stability of the current source over the full dynamic range required for tester operation, the speed of the current source must be reduced to less than ideal.
FIG. 1 shows a conventional current source 10 configured as a voltage-to-current converter. The current source 10 utilizes a conventional operational amplifier 12 to drive an n-channel power MOSFET device 14. The drain current of the power MOSFET 14 is sensed by a sense resistor, represented in FIG. 1 by resistor R.sub.sense.
Power MOSFETs, however, have several characteristics that make them difficult to use in the "linear" mode. First, power MOSFETs have large gate capacitances (approximately 800 pf for a 20 amp pulse output) which are proportional to the output current. Therefore, they require relatively large gate currents to turn on or off fast. Also, in the linear mode, power MOSFETs are prone to parasitic oscillation (self-oscillation) at a frequency of about 100 MHz. This behavior can be modeled as a simple grounded gate oscillator where the sense resistor Rsense is effectively the load.
As further shown in FIG. 1, one technique for compensating for the above-mentioned deficiencies of power MOSFETs in current source applications is to provide a series resistor R.sub.g between the output of op amp 12 and the gate of the power MOSFET 14. The resistor R.sub.g both isolates the capacitive load of the MOSFET gate from the operational amplifier 12 and reduces the "Q" (gain) of the MOSFET 14, preventing parasitic oscillation. Inclusion of the series resistor R.sub.g does not completely solve the oscillation problem, however, because it takes away the phase error margin of the operational amplifier 12 by increasing the feedback delay to the op amp's negative input. Even if a small value series resistor R.sub.g is used, the resistor R.sub.g in combination with the large gate capacitance of a power MOSFET 14 results in a large RC delay component. In addition, Miller capacitance resulting from the inclusion of the series resistor R.sub.g adds to the feedback delay. This increased delay almost always exceeds the phase margin of the operational amplifier 12.
To retrieve the phase margin, a capacitor C is typically added between the output of the operational amplifier 12 and its negative input. However, the added capacitor C slows down the overall response time of the operational amplifier 12, causing it to lose all high frequency response. In addition, the capacitor C must be large to handle the wide range (typically 10 volts-200 volts) of the load.
It would, therefore, be highly desireable to have available a high power current source that remains stable during high speed operation.